Existing wireless communications networks, such as a wireless Local Area Networks (LANs), contain a multitude of wireless communication devices (e.g., cellular telephones, personal digital assistants, laptop computers) located within a relatively small geographical area and that simultaneously communicate with the same wireless access point. The devices operate on Radio Frequency (RF) channels, the physical resources over which information is passed between the devices. Generally, in both analog and digital wireless communications, a desired RF signal of suitable frequency is modulated by a modulation signal that represents information, thereby generating a modulated RF signal. The modulated RF signal is transmitted to a radio receiver on the desired or selected RF channel.
When this modulated RF signal is received by a receiver, it is typically processed by the receiver prior to being converted to a digital signal representing the modulated information for further digital processing. This processing of the received RF signal may include combining the received RF signal with a local oscillator (LO) to produce an output signal which is subsequently digitally sampled by an Analog-to-Digital Converter (ADC). The process of combining the received RF signal with a LO signal to produce an output signal is referred to as mixing, down-converting, up-converting, down-mixing, up-mixing or modulating the RF signal with the LO signal. The receiver is a Direct Conversion Receiver (DCR) if the LO frequency is approximately equal to the frequency of the desired received RF signal, and the output signal produced by mixing the RF and the LO signals is a baseband signal (approximately 0 Hz). The receiver is a dual conversion receiver if the LO is offset from the desired received RF signal by an offset frequency typically referred to as an Intermediate Frequency (IF). Dual Conversion receivers typically incorporate additional IF stages to process the IF signal prior to being digitally sampled by the ADC.
Direct conversion receiver (DCR) architectures are generally desirable because they eliminate additional components of intermediate frequency stages, reducing the complexity and cost of the receiver. However, DCRs also suffer from problems that are more easily mitigated by using other types of receivers. One problem that is intrinsic to DCR architectures is baseband DC offset errors or DC distortion effects. Several sources of baseband DC offset errors exist in a DCR; one of which is caused by leakage of the LO signal to the inputs of mixers used to downconvert the RF signals, thereby creating a DC offset error associated with downconversion. DC offsets errors can also be caused by power variations of adjacent channel or off-channel RF signals, creating interference and producing a different DC offset error due to the non-ideality of the mixer.
The total DC offset error encountered is random and may vary depending on the receiver's operating environment, including temperature, changes in supply voltage to the physical circuits, and proximity to off-channel interference signals. The DCR may include circuit blocks that do not function properly in the presence of the undesired baseband DC offset errors. To minimize these DC offset errors, training systems may be incorporated into the DCR that utilize both digital and analog circuits that momentarily configure the DCR to a known training state to achieve optimal post-training performance. One such stage that must have minimum DC offset errors and minimum low frequency distortion to function properly is a Carrier Detect (CD) processing block which functions to detect the presence of a specific RF frequency at weak receive signal power levels.
Accordingly, there is a need for synchronizing a CD processing block with complementary receiver training systems to minimize undesired distortion artifacts while still providing proper processing of the digitized receive samples.